Photovoltaic generator with a spherical imaging lens for use with a paraboloidal solar reflector

Abstract

The invention is a generator for photovoltaic conversion of concentrated sunlight into electricity. A generator according to the invention incorporates a plurality of photovoltaic cells and is intended for operation near the focus of a large paraboloidal reflector pointed at the sun. Within the generator, the entering concentrated light is relayed by secondary optics to the cells arranged in a compact, concave array. The light is delivered to the cells at high concentration, consistent with high photovoltaic conversion efficiency and low cell cost per unit power output. Light enters the generator, preferably first through a sealing window, and passes through a field lens, preferably in the form of a full sphere or ball lens centered on the paraboloid focus. This lens forms a concentric, concave and wide-angle image of the primary reflector, where the intensity of the concentrated light is stabilized against changes in the position of concentrated light entering the generator. Receiving the stabilized light are flat photovoltaic cells made in different shapes and sizes and configured in a concave array corresponding to the concave image of a given primary reflector. Photovoltaic cells in a generator are also sized and interconnected so as to provide a single electrical output that remains high and stable, despite aberrations in the light delivered to the generator caused by, for example, mispointing or bending of the primary reflector. In some embodiments, the cells are set back from the image formed by the ball lens, and part of the light is reflected onto each cell small secondary reflectors in the form of mirrors set around its perimeter.

1. An apparatus for generating electricity from solar radiation, comprising:
 a dish-shaped reflector, said dish-shaped reflector having a focus and being adapted to concentrate solar radiation at said focus; 
 a substantially spherical lens positioned near the focus of said dish-shaped reflector; 
 a plurality of photovoltaic cells positioned at a receiving surface located in the optical path beyond said substantially spherical lens, said photovoltaic cells being operative to generate electricity when said photovoltaic cells are illuminated with solar radiation; and, 
 wherein said dish-shaped reflector is operative to reflect solar radiation to said substantially spherical lens, said substantially spherical lens is operative to spread the solar radiation to said photovoltaic cells, said photovoltaic cells being operative to generate electricity when solar radiation is focused upon said photovoltaic cells by said dish-shaped reflector and said substantially spherical lens. 
 
     
     
       2. The apparatus for generating electricity according to  claim 1 , wherein:
 said plurality of photovoltaic cells are configured to form a concave array positioned around said substantially spherical lens, said plurality of photovoltaic cells being substantially centered on the focus of said dish-shaped reflector. 
 
     
     
       3. The apparatus for generating electricity according to  claim 2 , wherein:
 a plurality of secondary reflectors are positioned around each of said plurality of photovoltaic cells, each of said plurality of secondary reflectors being configured in corresponding relationship with an associated photovoltaic cell selected from said plurality of photovoltaic cells to reflect solar radiation onto the associated photovoltaic cell, said plurality of secondary reflectors being arranged wherein a first secondary reflector selected from said plurality of secondary reflectors is positioned adjacent to a second secondary reflector selected from said plurality of secondary reflectors, each photovoltaic cell having one or more photovoltaically active areas and one or more inactive areas that do not generate electricity, each of said secondary reflectors being positioned in an inward-sloping configuration relative to the associated photovoltaic cell in order to reflect solar radiation onto a photovoltaically active area of the associated photovoltaic cell that would otherwise be lost on an inactive area that does not generate electricity, where the inward-sloping secondary reflectors for adjacent photovoltaic cells are joined together without substantial gaps so that substantially all of the solar radiation directed toward adjacent photovoltaic cells is reflected onto a photovoltaically active area of one of said photovoltaic cells. 
 
     
     
       4. The apparatus for generating electricity according to  claim 2 , further comprising:
 a plurality of secondary reflectors positioned around an associated photovoltaic cell selected from said plurality of photovoltaic cells, the associated photovoltaic cell having one or more photovoltaically active areas and one or more inactive areas that do not generate electricity, each secondary reflector being positioned in an inward-sloping configuration relative to the associated photovoltaic cell in order to reflect solar radiation onto a photovoltaically active area of the associated photovoltaic cell that would otherwise be lost on an inactive area that does not generate electricity. 
 
     
     
       5. The apparatus for generating electricity according to  claim 4 , wherein:
 the plurality of secondary reflectors completely surround the associated photovoltaic cell. 
 
     
     
       6. An apparatus for generating electricity from solar radiation, comprising:
 a dish-shaped primary reflector adapted to concentrate solar radiation at a focus, said dish-shaped primary reflector having a reflective surface with an axis of symmetry and a substantially paraboloidal curvature; 
 a substantially spherical lens positioned near the focus of said dish-shaped primary reflector which refracts the concentrated solar radiation to form an image of said dish-shaped primary reflector, said image coming to a focus on a concave image surface that is approximately concentric with the lens, the position of said image on said concave image surface being substantially stabilized against mispointing of said dish-shaped primary reflector away from the sun, the illumination of said image being locally substantially uniform, and the rays forming said image being directed approximately normal to said concave image surface, when the dish-shaped primary reflector is aligned to the sun; 
 a plurality of photovoltaic cells in a concave array to receive the solar radiation forming said image, and to generate electricity when illuminated with solar radiation; 
 secondary reflectors located between said substantially spherical lens and said photovoltaic cells, said secondary reflectors being adapted to aid in reflecting solar radiation from said substantially spherical lens onto said photovoltaic cells; and, 
 wherein said dish-shaped primary reflector is adapted to reflect solar radiation to said substantially spherical lens; said substantially spherical lens is adapted to spread the solar radiation to said photovoltaic cells; and said photovoltaic cells being adapted to generate electricity when solar radiation is focused upon said photovoltaic cells by said dish-shaped primary reflector and said substantially spherical lens. 
 
     
     
       7. The apparatus for generating electricity according to  claim 6 , wherein:
 said dish shaped primary reflector has a perimeter which is substantially square, when projected along said axis of symmetry of said dish-shaped primary reflector. 
 
     
     
       8. The apparatus for generating electricity according to  claim 6 , wherein:
 said substantially spherical lens forms a hermetically sealed entrance window to a chamber formed between said substantially spherical lens and said photovoltaic cells, said chamber surrounding and protecting said photovoltaic cells and said secondary reflectors against contamination. 
 
     
     
       9. The apparatus for generating electricity according to  claim 6 , wherein:
 said photovoltaic cells are individually substantially flat, and are tilted with respect to each other so as to form said concave array. 
 
     
     
       10. The apparatus for generating electricity according to  claim 6 , wherein:
 said photovoltaic cells are densely packed as a mosaic of photovoltaic cells, and the mosaic has a shape and size that substantially coincides with the shape and size of said image of said dish-shaped primary reflector formed by said substantially spherical lens, such that a large fraction of the concentrated solar radiation refracted by said substantially spherical lens falls directly on said photovoltaic cells. 
 
     
     
       11. The apparatus for generating electricity according to  claim 6 , wherein:
 a plurality of said photovoltaic cells each have a photovoltaically active area and the photovoltaically active areas are of different sizes; 
 wherein the plurality of photovoltaic cells are arrayed with photovoltaically active areas decreasing in area with increasing distance from said axis of symmetry in order to compensate for differences in the intensity of concentrated solar radiation focused upon each of the plurality of photovoltaic cells, so that each of the plurality of photovoltaic cells receives substantially the same power and generates substantially the same electrical current. 
 
     
     
       12. The apparatus for generating electricity according to  claim 6 , wherein:
 photovoltaic cells that generate substantially the same current are electrically connected in series. 
 
     
     
       13. The apparatus for generating electricity according to  claim 6 , further comprising:
 bypass diodes located outside the region of concentrated solar radiation that forms said image of said dish-shaped primary reflector, said bypass diodes being electrically connected in parallel with corresponding photovoltaic cells to provide an electrical bypass of the corresponding photovoltaic cells when a corresponding photovoltaic cell fails to generate adequate current. 
 
     
     
       14. The apparatus for generating electricity according to  claim 6 , wherein:
 said secondary reflectors comprise substantially continuous, inward-sloping reflecting surfaces arranged as a mirrored frame, each of the mirrored frames being positioned around a corresponding photovoltaic cell selected from said plurality of photovoltaic cells, each corresponding photovoltaic cell having a photovoltaically active area and one or more light insensitive regions; 
 the edge of said substantially continuous, inward-sloping reflecting surfaces of each mirrored frame nearest the substantially spherical lens forming a substantially planar entrance aperture with a substantially continuous perimeter, said entrance aperture being located substantially tangent to the surface of said concave image surface formed by said substantially spherical lens of said dish-shaped primary reflector; 
 the edge of said substantially continuous, inward-sloping reflecting surfaces of each mirrored frame nearest the corresponding photovoltaic cell forming a substantially planar exit aperture with a substantially continuous perimeter located close to the photovoltaically active area of the corresponding photovoltaic cell, said exit aperture being smaller than said entrance aperture and configured to match closely in shape and size the photovoltaically active area of the corresponding photovoltaic cell; 
 wherein each mirrored frame is adapted to relay solar radiation from a section of said image formed by said substantially spherical lens onto the photovoltaically active area of the corresponding photovoltaic cell that would otherwise impinge on a light-insensitive region and be wasted; and 
 wherein said mirrored frames are configured adjacent to each other and space is provided between adjacent mirrored frames for electrical components and connections between said plurality of photovoltaic cells. 
 
     
     
       15. The apparatus for generating electricity according to  claim 14 , wherein:
 said substantially continuous, inward-sloping reflecting surface has a substrate made of a thermally conductive material, and said substrate is mounted to provide a thermal path to conduct away heat absorbed from concentrated solar radiation incident on said surface. 
 
     
     
       16. The apparatus for generating electricity according to  claim 14 , where:
 said electrical components include a plurality of bypass diodes, each bypass diode being connected electrically in parallel with one of said plurality of photovoltaic cells and being located in said space between adjacent mirrored frames. 
 
     
     
       17. The apparatus for generating electricity according to  claim 14 , wherein:
 said dish-shaped primary reflector has a perimeter which is substantially square, and the entrance aperture of each mirrored frame is quadrilateral, the four corners of said quadrilateral entrance apertures being defined by rays, said rays originating from a flat regular square grid set before said dish-shaped primary reflector, said grid having the same square aperture as said dish-shaped primary reflector and oriented normal to said axis of symmetry; 
 wherein said rays are projected parallel to said symmetry axis onto said dish-shaped primary reflector, where they are reflected to pass undeviated through the center of said substantially spherical lens, and which terminate on said concave image surface formed by said substantially spherical lens of said dish-shaped primary reflector; and 
 wherein said quadrilateral entrance apertures are approximately but not exactly square and at least one said entrance aperture has a different size compared with at least one other of said quadrilateral entrance apertures, and wherein said quadrilateral entrance apertures intercept substantially the same optical power when said dish-shaped primary reflector is oriented with said axis of symmetry directed toward the sun. 
 
     
     
       18. The apparatus for generating electricity according to  claim 17 , wherein:
 said dish-shaped primary reflector has a perimeter which is substantially square; 
 the photovoltaically active area of each corresponding photovoltaic cell is substantially square; 
 the exit aperture of each said mirrored frame being substantially square, the substantially square exit aperture of each mirrored frame closely matching the substantially square photovoltaically active area of the corresponding photovoltaic cell; and, 
 each said mirrored frame comprising four of said substantially continuous, inward-sloping reflecting surfaces, said four reflecting surfaces of each said mirrored frame meeting along four corners, said four reflecting surfaces being generally non-planar, said four reflecting surfaces comprising a plurality of slightly tilted planar facets, where each of said four reflecting surfaces links one side of the approximately but not exactly square entrance aperture of each said mirrored frame to the corresponding side of the substantially square exit aperture of the mirrored frame. 
 
     
     
       19. The apparatus for generating electricity according to  claim 18 , wherein:
 said four substantially continuous, inward-sloping reflecting surfaces are configured to substantially uniformly distribute solar radiation falling on a photovoltaic cell, by angling the reflecting surfaces inward such that rays of concentrated solar radiation pass close to the center of said substantially spherical lens when said dish-shaped primary reflector is aligned with said axis of symmetry aimed at the sun, and wherein such rays which are reflected once by a reflecting surface will extend from an edge to approximately half way across the substantially square photovoltaically active areas of said photovoltaic cells. 
 
     
     
       20. The apparatus for generating electricity according to  claim 19 , wherein:
 the solar radiation falling on the substantially square photovoltaically active area of a photovoltaic cell has a concentration that is increased from said entrance aperture to said exit aperture of said mirrored frame by a factor greater than two. 
 
     
     
       21. The apparatus for generating electricity according to  claim 6 , wherein:
 said photovoltaic cells are electrically connected in parallel in radially-oriented groups, wherein the photovoltaically active areas of the photovoltaic cells are configured so that each such group of photovoltaic cells connected electrically in parallel produce substantially the same photovoltaic current when said dish-shaped primary reflector is pointed at the sun. 
 
     
     
       22. The apparatus for generating electricity according to  claim 21 , wherein:
 each such group of photovoltaic cells are electrically connected in parallel in radially-oriented groups so that cell current differences are spatially averaged when said dish-shaped primary reflector is mispointed away from the sun. 
 
     
     
       23. The apparatus for generating electricity according to  claim 21 , wherein:
 a plurality of such groups of photovoltaic cells each includes at least one photovoltaic cell located at the perimeter of said concave image surface and at least one adjacent photovoltaic cell located inward toward the center of said concave image surface to minimize changes in the current through the photovoltaic cells of such groups when said dish-shaped primary reflector is mispointed away from the sun as compared to the current through the photovoltaic cells of such groups when said dish-shaped primary reflector is pointed directly at the sun. 
 
     
     
       24. The apparatus for generating electricity according to  claim 21 , further comprising:
 bypass diodes electrically connected to such groups of photovoltaic cells to provide conductivity at times that a photovoltaic cell is not sufficiently illuminated. 
 
     
     
       25. The apparatus for generating electricity according to  claim 21 , wherein:
 a plurality of such groups of photovoltaic cells yielding substantially the same photovoltaic current are connected in series. 
 
     
     
       26. The apparatus for generating electricity according to  claim 6 , wherein:
 said photovoltaic cells in the concave array are each mounted individually on a substantially flat ceramic circuit board. 
 
     
     
       27. The apparatus for generating electricity according to  claim 26 , further comprising:
 a bypass diode located on each said ceramic circuit board and electrically connected to at least one corresponding photovoltaic cell. 
 
     
     
       28. The apparatus for generating electricity according to  claim 26 , wherein:
 said ceramic circuit boards are configured in a circular radial tiling pattern to substantially conform to said concave image surface. 
 
     
     
       29. The apparatus for generating electricity according to  claim 6 , further comprising:
 a shell faceted in a circular radial tiling pattern, said photovoltaic cells each being affixed to an individual corresponding substantially flat facet of said shell by a thermally conductive material, said shell comprising a composite structure having a copper layer, said shell forming part of a chamber containing cooling fluid, said thermally conductive material being configured to facilitate heat transfer from said photovoltaic cells to said copper layer, and said copper layer being configured to provide a thermal path to said cooling fluid. 
 
     
     
       30. The apparatus for generating electricity according to  claim 29 , wherein:
 said copper layer includes protruding pins to facilitate heat transfer into said cooling fluid. 
 
     
     
       31. The apparatus for generating electricity according to  claim 29 , wherein:
 said shell comprises a ceramic circuit board. 
 
     
     
       32. The apparatus for generating electricity according to  claim 6 , further comprising:
 a ceramic shell faceted in a circular radial tiling pattern having an inner concave side and an outer convex side, said photovoltaic cells each being affixed to an individual corresponding substantially flat facet on the inner concave side of said shell by a thermally conductive material, said shell comprising a composite structure having a copper layer, the outer convex side of said shell forming part of a chamber containing cooling fluid, said thermally conductive material and said copper layer providing a thermal path to said cooling fluid. 
 
     
     
       33. The apparatus for generating electricity according to  claim 32 , wherein:
 said ceramic shell includes pins protruding from the outer convex side of said ceramic shell into said cooling fluid to facilitate heat transfer into said cooling fluid. 
 
     
     
       34. An apparatus for generating electricity from solar radiation, comprising:
 a dish-shaped primary reflector adapted to concentrate solar radiation at a focus, said dish-shaped primary reflector having a reflective surface with an axis of symmetry and a substantially paraboloidal curvature; 
 a substantially spherical lens positioned near the focus of said dish-shaped primary reflector which refracts the concentrated solar radiation to form an image of said dish-shaped primary reflector, said image coming to a focus on a concave image surface that is approximately concentric with the lens, the position of said image on said concave image surface being substantially stabilized against mispointing of said dish-shaped primary reflector away from the sun, the illumination of said image being locally substantially uniform, and the rays forming said image being directed approximately normal to said concave image surface, when the dish-shaped primary reflector is aligned to the sun; 
 a plurality of photovoltaic cells in a concave array to receive the solar radiation forming said image, and to generate electricity when illuminated with solar radiation; and, 
 wherein said dish-shaped primary reflector is adapted to reflect solar radiation to said substantially spherical lens; said substantially spherical lens is adapted to spread the solar radiation to said photovoltaic cells; and said photovoltaic cells being adapted to generate electricity when solar radiation is focused upon said photovoltaic cells by said dish-shaped primary reflector and said substantially spherical lens. 
 
     
     
       35. The apparatus for generating electricity according to  claim 34 , further comprising:
 a plurality of lightpipes having an entrance aperture for receiving solar radiation from said substantially spherical lens, each said lightpipe having an exit aperture corresponding to a photovoltaic cell in the concave array for illuminating said corresponding photovoltaic cell with solar radiation. 
 
     
     
       36. An apparatus for generating power from solar radiation, comprising:
 a dish-shaped reflector, said dish-shaped reflector having a focus and being adapted to concentrate solar radiation at said focus; 
 a tracker supporting said dish-shaped reflector, said tracker being movable and operative to orient said dish-shaped reflector to receive solar radiation from the sun and to track the position of the sun during daylight hours; 
 a ball lens positioned near the focus of said dish-shaped reflector, said ball lens being supported by said tracker; and, 
 a generator configured to receive sunlight reflected from said dish-shaped reflector through said ball lens, said generator including a concave array of a plurality of photovoltaic cells, said ball lens being operative to form a concave image of the dish-shaped reflector upon the concave array of photovoltaic cells, said generator being supported by said tracker, said generator being operative to convert solar radiation into usable power. 
 
     
     
       37. The apparatus for generating power according to  claim 36 , further comprising:
 a protective window positioned between said dish-shaped reflector and said ball lens, said protective window and said ball lens forming a sealed enclosure to prevent contamination of the ball lens. 
 
     
     
       38. The apparatus for generating power according to  claim 37 , wherein:
 said protective window is made substantially larger in area than said ball lens, so that the intensity of concentrated solar radiation at said protective window is substantially less than at said ball lens. 
 
     
     
       39. The apparatus for generating power according to  claim 36 , wherein:
 said ball lens comprises fused silica glass. 
 
     
     
       40. An apparatus for generating power from solar radiation, comprising:
 a tracker for tracking the movement of the sun, said tracker having a base support, said tracker comprising a movable steel structure supported by said base support; 
 a plurality of dish-shaped reflectors, each said dish-shaped reflector having a focus and being adapted to concentrate solar radiation at said focus, said dish-shaped reflectors being supported by the steel structure of said tracker in a co-axial array; 
 a ball lens positioned near the focus of each said dish-shaped reflector and supported by the steel structure of said tracker; and, 
 a generator configured to receive sunlight from said ball lens, said generator comprising a plurality of photovoltaic cells configured in a concave array around said ball lens, said ball lens being adapted to provide substantially uniform illumination of the plurality of photovoltaic cells, said generator being supported by the steel structure of said tracker, said generator being operative to convert solar radiation into usable power; 
 whereby power is generated from solar radiation by the generators when the array of dish-shaped reflectors, and corresponding ball lenses and generators, are pointed at the sun. 
 
     
     
       41. The apparatus for generating power according to  claim 40 , wherein:
 the ball lens is a full sphere. 
 
     
     
       42. The apparatus for generating power according to  claim 40 , wherein:
 the generator includes secondary reflectors located between the ball lens and the concave array of photovoltaic cells, the secondary reflectors being adapted to aid in reflecting solar radiation from the ball lens onto the photovoltaic cells. 
 
     
     
       43. The apparatus for generating power according to  claim 42 , wherein:
 the ball lens is a full sphere. 
 
     
     
       44. The apparatus for generating power according to  claim 43 , wherein:
 the dish-shaped reflectors are paraboloidal reflectors. 
 
     
     
       45. The apparatus for generating power according to  claim 44 , wherein:
 the tracker is a two-axis tracker.